1. Field of the Invention
The present invention relates to an acousto-optic modulator, and more particularly, to an acousto-optic modulator having two transducers for facilitating impedance matching.
2. Description of Background Art
Various display technologies have been developed to meet the increasing demand for multimedia applications. However, well known conventional devices such as cathode ray tubes (CRTs) or liquid crystal displays (CDs) have not provided satisfactory performance in large screen applications due to manufacturing difficulties and because of their poor resolution. Conventional large screen video projection devices suffer similar disadvantages because they also utilize CRTs or LCDs.
An alternative video system utilizes a laser projector for projecting video information directly onto a screen. The advantages of the laser projector in large screen applications include a large display area, high contrast, high light efficiency, minimal distortion and color errors, the ability to provide luminance and contrast irrespective of projection distance, and compatibility with large-screen HDTV (High Definition Television) format. Laser projectors generally employ either a xenon (Xe) lamp, a helium-neon (He-Ne) laser, an argon (Ar) laser or a krypton-argon (Kr-Ar) laser as a light source. Amongst these light sources, focus has been placed on the Kr-Ar laser for its ability to simplify the projection system.
In general, a laser projection system has a light generator, a light modulator, a video signal generator, a scanner and a screen. A beam generated by the laser generator is directed through the light modulator, which modulates the incident beam in accordance with a video signal supplied by the video signal generator. The modulator operates to load pixel information onto the beam. The modulated beam is scanned onto the screen by the scanner, thereby displaying a picture on the screen.
Generally, three types of light modulators are known for laser projection systems: an acousto-optic modulator (AOM), an electro-optic modulator (EOM), and an acoustooptic tunable filter (AOTF). Among them, the AOM is the most widely used, and the driving circuit thereof is simple. For large screen displays, the conventional light modulating system uses three AOMs in conjunction with a high power laser beam. The AOMs used in such systems generally have a structure that can provide sustained operation at high power.
A conventional AOM is shown in FIG. 1. Referring to FIG. 1, the conventional AOM has a transducer 12 for generating acoustic waves by an electrode 14 in one side of an ultrasonic medium 10. The transducer 12 is coupled to the ultrasonic medium 10 by an adhesion layer 13 having a silver (Ag) layer, and an electrode 14 made of gold (Au) is installed on the transducer 12. The transducer 12 is thus constructed in such a way that it is interposed between the electrode 14 and the adhesion layer 13 and serves as a capacitor.
An acoustic wave absorbing medium (not shown) is provided at the opposite side of the transducer 12 to prevent reflection of ultrasonic waves back through the medium 10. Laser light enters the ultrasonic medium 10 at surface 10A and is emitted from the medium at surface 10B. Surfaces 10A and 10B are optically abraded. Since the ultrasonic medium 10 has a large refractive index, much of the reflected laser light that enters through the light receiving surface 10A and is emitted through light emitting surface 10B is lost. Thus, a film that prevents laser light reflection is generally deposited on the light receiving and emitting surfaces 10A and 10B.
FIG. 2 is a plan view of the electrode 14 of a transducer 12 of a conventional AOM. Referring to FIG. 2, the electrode 14 comprises a single element. Assuming that the length (1) of the electrode 14 is 10 mm and the average height thereof (h.sub.1 +h.sub.2)/2 is 0.55 mm, the sectional surface area (A) of the electrode 14 is 5.5 mm.sup.2.
To use the AOM together with a driver, an impedance matching operation must be performed. The goals of the impedance matching operation are to compensate for frequency mismatch resulting from a thickness error of a transducer so that the transducer will resonate at the driver frequency, and to optimize the transfer of RF power by matching the impedance of the driver and transducer.
As explained below, in the case where a conventional driver having a conventional driver impedance (R.sub.s) of 50 .OMEGA. is used to drive an AOM at 150 MHZ, conventional AOMs are not able to match the 50 .OMEGA. driver impedance. As a result, conventional AOMs are not able to provide efficient transfer of RF power. Accordingly, there is a need for an acousto-optic modulator (AOM) for laser projection display systems that provides superior impedance matching.